1. Field of the Invention
The present invention relates to a developing method for developing either the electrostatic latent image, which is formed by the electronically photographic process, the electrostatic recording process, the electrostatic printing process or the like, or the latent image such as the magnetic latent image which is formed by the magnetic printing process, and a developing apparatus therefor.
2. Description of the Prior Art
As a system for developing an electrostatic latent image to form a visible image, generally speaking, there have been known to the prior art both a wet type developing system using a liquid developing agent and a dry type developing system using a powdered developing agent. The latter system, which resorts to the dry process so that it is superior in that it needs no volatile solvent, is further divided in a rough manner into two systems, i.e., the system using a two-component developing agent composed of a carrier and a toner and the system using a one-component developing agent composed only of a toner.
Thus, the developing system using the one-component developing agent is superior to the system using the two-component developing agent in that no change in the toner concentration essentially takes place to simplify the construction of the developer and in that the characteristics of the developing agent are not deteriorated but can be retained stable for a long time. In another aspect, however, the system using the one-component developing agent has a serious defect that the stable formability of a picture image cannot be attained. This defect is caused by the fact that it is difficult to charge the one-component developing agent to a desired state and to apply the developing agent at a desired state to an electrostatic charge retaining member when in the developing operation.
In order to attain an excellent visible image by the dry type developing system, more specifically, it is necessary to charge the toner such that it acquires a polarity opposite to that of the electrostatic latent image to be developed and such that it is brought into a state having a proper level. In the system using the two-component developing agent, the toner is frictionally charged by mechanically stirring the toner and the carrier so that the charged polarity and extent of the toner can be considerably controlled by selecting the characteristics of the carrier, the stirring condition and so on. In the one-component developing agent composed only of the toner, on the contrary, there exists no such carrier thereby to make it remarkably difficult to control the charged polarity and extent of the toner.
As one of the aforementioned developing methods using the one-component developing agent, there has been known in the prior art the so-called "powder cloud method" (which should be referred to the specification of U.S. Pat. No. 2,725,304). By blowing out the powders of a developing agent from a nozzle-shaped metal tube, according to that method, the developing agent particles are charged by the contacting friction, and the cloud of the developing agent is simultaneously formed at a sprayed state and is applied to an electrostatic latent image so that it may be developed. According to this method, the respective developing agent particles can migrate in accordance with the electric lines of force established by the electrostatic latent image, thus making it possible to attain a visible picture image having a high resolution.
In the prior art, however, not only since the charged state of the developing agent cannot be properly controlled, as has been described hereinbefore, but also since there is known no proper means for forming the cloud of the developing agent at a stable state and since the developing agent is scattered in accordance with the formation of the developing agent cloud thereby to raise a serious problem, the powder cloud method thus far described has been put into little practice. Although there has also been proposed another powder cloud method (which should be referred to Japanese Patent Publication Laid-Open to Public Inspection No. 73,067/78) using a two-component developing agent and taking the control of the charged state of the developing agent into consideration, the defects resulting from the use of the two-component developing agent such as the contamination due to the scatter of the toner or the fog in the electrostatic latent image retaining member are not improved in the least because the toner is scattered by mechanical means according to the method considered thereby to form the cloud.
On the other hand, the developing apparatus according to the prior art has defects that it is necessary to provide a developing agent charging mechanism, which is independent of the developing agent conveyor, so as to charge the developing agent so that the developing apparatus as a whole has its size considerably enlarged by that developing agent charging mechanism, and that it is difficult to sufficiently satisfy the requirement for a high positional accuracy or the like.